Various fuel pumps that are used to provide fuel to internal combustion engines are driven by a lobed camshaft that displaces a tappet assembly with a roller that rolls along the cam lobe. The reciprocating motion of the tappet assembly generates fuel flow that is used by the internal combustion engine to generate power. These types of fuel pumps are popular in modern fuel systems, and are frequently used in common rail fuel systems applications where high fuel pressures are required. In such fuel pumps, the reciprocating tappet assembly is guided so that the centerline of the roller and the centerline of the camshaft are maintained in an aligned position with respect to each other.
Conventional fuel pumps utilize a solid dowel pin that is pressed into the housing of the fuel pump from the exterior of the fuel pump housing to maintain the rotational positioning of the tappet assembly relative to the camshaft lobe. In this regard, the housing of the fuel pump is generally provided with a hole that extends through the housing so that the dowel pin can be inserted. The roller tappet assembly typically includes a key slot into which the dowel pin extends, thereby allowing reciprocal displacement of the tappet assembly while preventing rotation of the roller tappet assembly relative to the camshaft.
U.S. Pat. No. 5,415,533 issued to Egger et al. discloses another example system and method of guiding a roller tappet assembly. A cylindrical disk is situated as a guide piece of a radial bore in a slide bush, and is secured in place by a wire ring that is guided in an external annular groove in the slide bush. The cylindrical disk is inserted from the exterior of the fuel injection pump through an opening in the slide bush. The cylindrical disk is pressed against a planar recess which runs perpendicular to the axis of the pump piston. The horizontal edges of the recess constrain the movement of the roller tappet assembly in the slide bush, while the vertical edges of the recess prevent the rotational position of the roller tappet assembly.
U.S. Pat. No. 3,822,683 issued to Clouse discloses still another system and method of guiding a roller tappet assembly in which the cylindrical member of a plunger assembly is reciprocally mounted in a cylindrical bore. The reference discloses that rotation of the cylindrical member is prevented by a key that engages a longitudinal slot axially formed on the cylindrical member.
As described in further detail below, various limitations have been noted in the above described systems and methods for guiding a tappet assembly that negatively impact the durability and reliability of the fuel pump. Therefore, there exists an unfulfilled need for a fuel pump and methods that minimize these limitations, and increase the durability and reliability of the fuel pump.